The liquid portion of the blood consists of material formed from food, air, and water. From this material the cells of the blood are formed: first, the white cells, which are incomplete in formation; and then the red cells, which are completed by the addition of the oxygen received from air in the lungs. Fig. 49 represents part of a magnified blood-vessel, _a_, _a_, in which the round cells are the white, and the oblong the red cells, floating in the blood. Surrounding the blood- vessels are the cells forming the adjacent membrane, _bb_, each having a nucleus in its centre.

Cells have different powers of selecting and secreting diverse materials from the blood. Thus, some secrete bile to carry to the liver, others secrete saliva for the mouth, others take up the tears, and still others take material for the brain, muscles, and all other organs. Cells also have a converting power, of taking one kind of matter from the blood, and changing it to another kind. They are minute chemical laboratories all over the body, changing materials of one kind to another form in which they can be made useful.

Both animal and vegetable substances are formed of cells. But the vegetable cells take up and use unorganized or simple, natural matter; whereas the animal cell only takes substances already organized into vegetable or animal life, and then changes one compound into another of different proportions and nature.

These curious facts in regard to cell-life have important relations to the general subject of the care of health, and also to the cure of disease, as will be noticed in following chapters.

THE NERVOUS SYSTEM.

There is another portion of the body, which is so intimately connected with every other that it is placed in this chapter as also having reference to every department in the general subject of the care of health.

The body has no power to move itself, but is a collection of instruments to be used by the mind in securing various kinds of knowledge and enjoyment. The organs through which the mind thus operates are the _brain_ and _nerves_. The drawing (Fig. 50) represents them.

[Illustration: Fig. 50.]

The brain lies in the skull, and is divided into the large or upper brain, marked 1, and the small or lower brain, marked 2. From the brain runs the spinal marrow through the spine or backbone. From each side of the spine the large nerves run out into innumerable smaller branches to every portion of the body. The drawing shows only some of the larger branches. Those marked 3 run to the neck and organs of the chest; those marked 4 go to the arms; those below the arms, marked 3, go to the trunk; and those marked 5 go to the legs.

The brain and nerves consist of two kinds of nervous matter--the _gray_, which is supposed to be the portion that originates and controls a nervous fluid which imparts power of action; and the _white_, which seems to conduct this fluid to every part of the body.